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SiO2@GO nanohybrid membranes via thermally induced phase separation method
Author’s Accepted Manuscript Preparation and properties of PVDF/SiO2@GO nanohybrid membranes via thermally induced phase separation method Zhong-Kun Li, Wan-Zhong Lang, Wei Miao, Xi Yan, Ya-Jun Guo www.elsevier.com/locate/memsci
PII: DOI: Reference:
S0376-7388(16)30186-7 http://dx.doi.org/10.1016/j.memsci.2016.03.048 MEMSCI14386
To appear in: Journal of Membrane Science Received date: 2 December 2015 Revised date: 9 March 2016 Accepted date: 23 March 2016 Cite this article as: Zhong-Kun Li, Wan-Zhong Lang, Wei Miao, Xi Yan and YaJun Guo, Preparation and properties of PVDF/SiO2@GO nanohybrid membranes via thermally induced phase separation method, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2016.03.048 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Preparation and properties of PVDF/SiO2@GO nanohybrid membranes via thermally induced phase separation method Zhong-Kun Li, Wan-Zhong Lang*, Wei Miao, Xi Yan, Ya-Jun Guo The Education Ministry Key Laboratory of Resource Chemistry and Shanghai Key Laboratory of Rare Earth Functional Materials, Department of Chemistry and Chemical Engineering, Shanghai Normal University, 100 Guilin Road, Shanghai 200234, China. *
Corresponding author. Tel: +86 21 64321951; fax: +86 21 64321951. [email protected]
Abstract
Graphene oxide (GO), one of the most promising filter materials, is intensively employed to prepare the membranes via non-solvent induced phase separation(NIPS) method, but it hasn’t been found to modify polymeric membranes via thermally induced phase separation (TIPS) method. In this work, the derivative of GO, SiO2@GO nanohybrid, is fabricated and employed to synthesize PVDF/SiO2@GO nanohybrid membranes via TIPS method for the first time. The results indicate that the PVDF/SiO2@GO nanohybrid membranes experience liquid-liquid phase separation mechanism, and exhibit bi-continual and asymmetric structure. The included SiO2@GO nanohybrid is uniformly dispersed in the membrane matrix. With the addition of SiO2@GO, the top surface becomes denser and the pore size decreases; but overhigh SiO2@GO addition for membrane M-5 triggers in the adverse trend. This is caused by the combined actions of nucleation and growth of PVDF and viscosity increase of cast solution due to the addition SiO2@GO nanohybrid. The BSA rejection of membrane gradually increases with the addition of PVDF/SiO2@GO nanohybrid accompanied with the decline of pure water permeation flux. As the SiO2@GO content increases to 0.9wt.%, the PVDF/SiO2@GO nanohybrid membrane M-4 presents the highest BSA rejection of 91.7% and the lowest permeation flux of 182.6 L·m-2·h-1·bar-1. However, the overhigh SiO2@GO addition (1.2wt.%) leads to the outstanding pure water permeation flux of 679.1 L·m-2·h-1·bar-1 and a much lower BSA rejection. The addition of PVDF/SiO2@GO nanohybrid evidently improves the surface hydrophilicity and antifouling ability of resultant membranes. The XRD patterns and FTIR spectra of membranes verify the exclusive α-phase of PVDF, and the melting temperature(Tm) and crystallinity(xc) evidently increase with the addition of SiO2@GO nanohybrid up to 0.9wt.% in the dopes. Overhigh SiO2@GO addition (1.2wt.%) leads to small decline for 1
these parameters. Graphical Abstract Keywords: poly(vinylidene fluoride) (PVDF); graphene oxide (GO); ultrafiltration; thermally induced phase separation(TIPS); SiO2@GO nanohybrid
1. Introduction Membrane separation technology is widely used to remove various water contaminants, which have significant adverse health and/or environmental effects [1, 2]. Due to the outstanding properties such as high thermal stability, good chemical resistance, high mechanical strength, good aging resistance and membrane forming properties, poly(vinylidene fluoride)(PVDF) membranes were applied to a range of scientific researches and industrial processes[3, 4]. To date, most commercial and lab membranes are produced via a phase inversion process. Thermally induced phase separation (TIPS) method was recognized since the late 1980s; and from then on much more works were devoted to prepare porous membranes via TIPS method[5, 6]. Apart from the polymers such as polypropylene (PP)[7] and polyethylene (PE), the PVDF membranes[8-11] prepared by TIPS methods were found more significant in mechanical strength, pore size distribution and permeation performances compared with those produced by non-solvent induced phase separation(NIPS) method. In the past years, most works in membrane preparations via TIPS process were intensively concentrated on the selection and effects of diluents on the membrane structure and permeation performance [9, 12-18]. For instance, Atkinson et al.[12, 13] employed isotactic polypropylene (iPP) and diphenyl ether (DPE) to produce flat membranes with an anisotropic structure via TIPS, which underwent a liquid-liquid phase separation mechanism. Ji et al.[14] selected dibutyl phthalate (DBP) and di(2-ethylhexyl) phthalate (DEHP) as diluents to prepare PVDF hollow fiber membranes. In fact, the additives also markedly influence the membrane structure and performance in TIPS process, which is similar to NIPS process. Nevertheless, only a few works were paid on this side. For example, Li et al.[19] reported that CaCO3 influenced the crystallization kinetics of ternary mixture of PVDF/CaCO3/DBP in preparing membranes via TIPS. Cui et al.[11] found that a certain amount of micro-sized SiO2 promoted the water flux of PVDF/SiO2 blend membranes and induced the formation of spherulitic morphology. Xu et al.[10] fabricated the PVDF/oxidized multi-wall carbon nanotubes(O-MWNTs) flat membranes with bi-continuous 2
structures. Graphene and its derivatives(such as graphene oxide, GO) are considered as one of the most promising filter materials[20, 21]. Although they were intensively employed to prepare the membranes with high anti-fouling and rejection performances via non-solvent induced phase separation (NIPS) method [22-28], the graphene and its derivatives (such as GO) have not been found to modify polymeric membranes via TIPS method. The low thermal stability of GO may be one factor inhibiting its applications in TIPS process. In order to overcome the drawback of graphene and its derivatives, SiO2@GO nanohybrid was produced via in situ hydrolysis and condensation of TEOS on the surfaces of graphene oxide (GO) nanosheets, and then used to decorate PVDF membranes via TIPS. This study aims to synthesize PVDF/SiO2@GO nanohybrid membranes for the first time. The influences of SiO2@GO nanohybrid on the membrane structure and permeation performances were detailedly investigated upon a series of characterizations. 2. Experimental 2.1 Materials Poly(vinylidene fluoride) (PVDF, FR904) was purchased from Shanghai 3F New Material Co. Ltd. (China). Graphite powder (325 mesh) and tetraethoxysilane (TEOS, 98%) were purchased from Aladdin Chemistry Co. Ltd. (Shanghai, China). Ammonia solution (NH3·H2O, 25wt%), dibutyl phthalate (DBP), hydrochloric acid( HCl, 36~38wt%), H2SO4(98wt%) and absolute ethanol were provided by Shanghai Chemical Agent Co. Ltd.(China). Deionized water (DI) was produced by a reverse osmosis (RO) system. Bovine serum albumin (BSA, MW=67000) was purchased from Shanghai Bio Co. Ltd. (China). All chemicals were used without further purification. 2.2 Preparation of inorganic particles 2.2.1 Synthesis of GO nanosheets Graphene oxide was synthesized from oxidized natural graphite flakes by a modified Hummers method [29, 30]. In a typical procedure, 3.0g graphite flakes and 18.0g KMnO4 were added to a mixed H2SO4/H3PO4 (volume ratio=360:40) solution; then the mixture was stirred and reacted at 50 °C for 12h. After the reaction, the mixtures were cooled to room temperature and poured into the ice bath with adding 3ml 30% H2O2, and then the remaining solid material was washed with DI water, 30% HCl aqueous solution, and absolute ethanol in succession. The solid was dried in vacuum oven at room temperature and stored for further study. 2.2.2 Synthesis of SiO2@GO nanohybrid Silica nanoparticles were deposited on GO nanosheets by in situ hydrolysis and condensation of 3
TEOS[31]. In a typical procedure, 0.5 g GO was dispersed in 420 ml alcohol–water (5/1, v/v) solution by sonication for 1 h. After that, the pH of reaction mixture was adjusted to approximate 9.0 with ammonia solution, and then 5.5 mL TEOS was added into the solution drop by drop. After being vigorously dispersed in ultrasonic bath for 1.5 h, the mixture was stood for 24 h at room temperature (25 °C). Then, the SiO2@GO suspension was centrifuged, washed for 5 times with alcohol, dried in vacuum. The obtained SiO2@GO nanohybrid was stored for further study. 2.2.3 Characterizations of GO and SiO2@GO The synthesized GO nanosheets and GO-SiO2 nanohydrids were detected by Fourier-transform infrared spectroscopy (FTIR Electron Corp Nicolet 380, US) in the range of 4000–400 cm−1 at room temperature. The thermal stability of GO nanosheets and SiO2@GO nanohybrid was measured by a thermal gravimetric analyzer (Perkin-Elmer Pyris 6 TGA instrument) at a heating rate of 10°C/min from 35°C to 800°C under nitrogen atmosphere. The morphologies of GO nanosheets and SiO2@GO nanohybrid were detected with a field emission transmission electron microscope (FETEM, JEM-2010, Japan) at 200 kV. 2.3 Membranes 2.3.1 Membrane preparation The PVDF/SiO2@GO nanohybrid membranes were prepared via TIPS process. The predetermined amount of SiO2@GO nanohybrid was completely dispersed in the measured amount of DBP diluent under ultrasonic bath for 1 h. Then 25 g of PVDF powder was added into the mixing solution. The homogenous casting solution was obtained by vigorous stirring at 200 °C for 6 h. After fully degassing, the casting solution was uniformly spread onto a heated stainless steel plate (200°C) with a knife gap of 450μm, and then immediately immersed into quenching bath (ice water bath) until totally solidification. To eliminate diluent, the nascent membranes were immersed into ethanol liquid for 12 h, and then soaked in DI water to extract the residual ethanol. The resulted membranes were then stored in DI water for further study in details. The detailed preparation
parameters for the membranes are were listed in Table 1. The PVDF and SiO2@GO contents in Table 1 represent the ones in the casting solutions. Table 1 Compositions of dope solutions and preparation temperature for PVDF/SiO2@GO membranes Mass ratio Membrane no
PVDF(g)
DBP(g)
SiO2@GO(g)
4
Membrane preparation temperature (°C)
Quenching temperature (°C)
M-1 M-2 M-3 M-4 M-5
25 25 25 25 25
75 75 75 75 75
0 0.3 0.6 0.9 1.2
200 200 200 200 200
0 0 0 0 0
2.3.2 Characterizations of membranes The membrane surfaces were analyzed by the attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) (Thermo Electron Corp., Nicolet 380, USA). The morphologies of the as-prepared membranes were detected using a field emission scanning electron microscopy (FESEM, Hitachi S-4800, Japan) equipped with an energy-dispersive spectrometry (EDS) under an accelerating voltage of 5.0 kV. Before recording the cross-section SEM images, the membrane samples were fractured in a liquid nitrogen and then sputtered with gold for 120 seconds. The AFM images of top surfaces were performed on an atomic-force microscopy (AFM, BioScope TM, USA) with tapping mode. The surface was imaged at a scan size of 2 μm×2 μm at a speed of 2 Hz. The hydrophilicity of hybrid membrane was evaluated by water contact angle system (KRUSS DSA30 German) equipped with video at room temperature. The contact angle change of 3.0 μL water drop on the top surface was recorded in air. The crystalline properties of membranes were investigated by a wide-angle X-ray diffraction (WAXD, D/max-II B, Japan) in the 2θ range of 5–55°. The crystallinity and thermal properties of membranes were measured using a DSC (DSCQ100 TA Instruments USA) measurement in N2 atmosphere between 25°C and 280°C. The melting enthalpy (∆Hf) values of membranes were calculated according to the DSC curves. As a result, the crystallinity (xc) was calculated on the basis of the following equation (1):
xc
ΔH f ΔH *f
100%
(1)
where △Hf* is the melting enthalpy of the neat PVDF(104.7 J/g)[32, 33]. To investigate the protein adsorption, the membranes with a diameter of 7 cm were soaked in 1 g·L-1 BSA aqueous solution (PBS buffer solution as a solvent, pH=7) at room temperature for 24 h. The protein concentrations before and after adsorption were measured by an ultraviolet spectrophotometer (UV3600 Shimadzu, Japan) at 280 nm. The adsorption capacity of each membrane was calculated according to the concentration difference before and after adsorption. The pure water permeation flux (JW1) and BSA rejections of as-prepared membranes were carried out by a self-made lab device with a valid membrane area of 26.04 cm2. First, all samples were compressed at 2 bar 5
for 30min, then the JW1 values of membranes were measured at 25 °C at 1 bar using the following equation (2):
JW 1
V A t P
(2)
where V is the volume of penetrative water (L), A is the effective membrane area (m2), t is the running time (h), and P is the trans-membrane pressure (bar). Afterwards the 500 ppm BSA aqueous solution as the feed solution was forced to permeate the membrane for 30 min to assess the fouling performance. The concentrations of permeate and feed solutions were determined by an ultraviolet spectrophotometer (UV3600 Shimadzu, Japan) at 280 nm. The rejection of BSA (R, %) was defined using the following equation (3):
R (1
cP ) 100% cF
(3)
where CP and CF are the concentrations of protein in the permeate and feed solutions, respectively. The permeation flux for protein aqueous was recorded as Jp1. After the filtration of BSA solution, the membrane was washed with 500 ppm NaClO aqueous solution at 25 °C for 30 min. After being rinsed with DI water, the pure water flux of the washed membrane was re-measured again and recorded as Jw2. To analyze the antifouling property, several parameters were defined as followed. The flux recovery ratio (FRR) was calculated by equation (4):
FRR
JW 2 100% JW 1
(4)
The reversible fouling ratio (Rr), irreversible fouling ratio (Rir) and total fouling ratio (Rt) were described as following:
J P1 100% JW 1
(5)
J w2 J P1 100% JW 1
(6)
Rt 1
Rr
Rt Rr Ri r
(7)
3. Results and discussion 3.1 Characterizations of GO and SiO2@GO Fig. 1 (a) depicts the FTIR spectra of GO and SiO2@GO nanohybrid. The peak at 1720 cm-1, which is related to the C=O vibration of carboxylic groups of GO, vanishes in the spectra of SiO2@GO [24, 31]. It can 6
be ascribed to the conversion of C=O on GO surface to Si-O-C after the reaction with TEOS, which demonstrates the interconnection between GO and silica coating. The spectrum of SiO2@GO show two new peeks at 1090 cm-1 and 465 cm-1, which are attributed to the Si–O–Si asymmetric vibration and bending vibrations respectively[34]. Also, the peaks at 800 cm-1 and 950 cm-1 are ascribed to the Si-O-Si symmetric vibration[31] and the Si-OH stretching band respectively. The thermal stability of material is vital to the structure and performance for the membranes prepared by TIPS method. The TGA curves of GO and SiO2@GO are presented in Fig.1 (b). The TGA curve of GO shows a marked weight loss at ~200 °C, which is mainly attributed to the decomposition of oxygen-containing groups. This is the reason that the neat GO as additive isn’t suitably used for membrane preparation by TIPS method. Nevertheless, after coating with silica, the phenomenon of thermal weight loss at ~200 °C isn’t found, and the thermal stability of SiO2@GO nanohybrid is evidently improved. It demonstrates the feasibility of using SiO2@GO nanohybrid as membrane material in TIPS process. The TEM image in Fig.1 (c) illustrates the ultra-thin and silk-like GO nanosheets, which indicates the successful chemical exfoliation of graphite via the modified Hummer’s method in this work. The TEM image of SiO2@GO in Fig.1 (d) shows that silica coating uniformly covers on GO nanosheets, which demonstrates clearly in situ hydrolysis and condensation of TEOS on the GO nanosheets.
Fig.1 (a) FTIR spectra of the GO and SiO2@GO; (b)TGA curves of the GO and SiO2@GO; (c)TEM image of 7
GO nanosheets; (d)TEM image of SiO2@GO nanosheets. 3.2 The morphologies of PVDF/SiO2@GO nanohybrid membranes From Figs.2-3, the top surfaces and bottom surfaces of PVDF/SiO2@GO nanohybrid membranes exhibit the outstanding fibrous and sheaf-like structures respectively, which are the characteristic morphology induced by the mechanism of liquid-liquid phase separation[10]. The top surfaces of membranes turn to denser and pore size becomes smaller with the loading of SiO2@GO until 0.9wt.% loading in the dope. However, the addition of SiO2@GO further increases to 1.2wt.% in the dope for membrane M-5, the top surface grows loser and pore size become larger. These phenomena may be explained that the introduction of SiO2@GO plays a typical role of nucleus and promotes the crystallization of PVDF. The crystallization process induces the different morphologies and pore sizes of the top surfaces. However, from Fig. S1, the addition of GO has little effect on the structure of PVDF/GO hybrid membranes, which may be due to the decomposition of GO sheets at high temperature in TIPS process. However, the bottom surfaces are evidently looser than the top surfaces, which may be ascribed to the combined effects of two sides: the loss of diluent near the top surface and the slower cooling rate near the bottom surface during the membrane preparation. The former factor leads to the higher polymer concentration before solidification and thus denser top surfaces. The later factor leaves the enough time for the mass transport between polymer-rich phase and polymer-lean phase until the final solidification and leads to the more porous bottom surfaces. Furthermore, it seems that the bottom surfaces present the similar structures, which are resulted from the heat preservation of the stainless steel plate in membrane preparation process.
M-3
M-2
M-1
M-4
M-5
8
Fig.2 The top surfaces of PVDF/SiO2@GO hybrid membranes
M-1
M-2
M-3
M-4
M-5
Fig.3 The bottom surfaces of PVDF/SiO2@GO hybrid membranes
M-1
M-2
M-3
9
M-4
M-5
Overall
Near top surface
Near bottom surface
Fig.4 The cross-sectional morphologies of PVDF/SiO2@GO hybrid membranes M-4
M-5 M-1
Fig.5 The EDS mapping of the M-4 and M-5(Si mapping) As shown in Fig.4, the cross-sectional morphologies of PVDF/SiO2@GO hybrid membranes present the asymmetric bi-continuous sponge-like structures without spherulites, which can further demonstrate that the membranes are dominated by liquid-liquid phase separation mechanism. This structure provides the base for high permeability and high mechanical strength of the membranes[16]. Furthermore it is interesting that the part near the top surface of membrane becomes denser and the part near the bottom surface turns to be looser with the loading of SiO2@GO, which have been discussed above. Moreover, the formation mechanism asymmetric bi-continuous sponge-like structures without spherulites could be analyzed by nucleation and growth of PVDF crystal induced by a TIPS process. In details, three principal accounts are involved: (i) liquid-liquid phase separation is initiated by temperature gap; meanwhile the loaded SiO2@GO nanohybrid acts as nucleuses and induce the nucleation and growth of PVDF crystals; (ii) upon substantial increasing the addition of SiO2@GO nanohybrid, more nucleus favors the formation of more crystalline grains, and consequently produces the smaller pores onto the top surface; (iii) overhigh SiO2@GO content brings about time shortage to crystal growing owing to viscosity increase. Hence the crystalline decreases and causes the 10
bigger pores and higher porosity. The FESEM images of top surfaces in Fig. 1 also prove the hypothesis above. Xu et al.[10] also observed the similar phenomena in PVDF membranes blended with O-MWNTs via TIPS method. Cui et al.[11] verified the effects of nucleation and growth on the structure and performance PVDF membranes, which were prepared by the ternary mixture of PVDF/SiO2/DBP via TIPS process. The results also demonstrated that the membrane surface became denser with the addition of micro-particals. The dispersion of SiO2@GO in membrane matrix also influences the morphology and membrane performances. Fig.5 illustrates that Si species is homogeneously dispersed in the cross-sectional part of membranes due to good compatibility.
Fig.6 AFM images of the top surfaces of PVDF/SiO2@GO nanohybrid membranes 11
Table 2 The surface roughness parameters of PVDF/SiO2@GO nanohybrid membranes Membrane no.
Ra(nm)
Rq(nm)
Rmax(nm)
M-1 M-2 M-3 M-4 M-5
41.223 23.247 17.588 17.975 20.082
52.951 29.65 22.101 22.822 25.646
368.24 238.93 165.18 179.72 209.17
The top surfaces are also detected by AFM technique and shown in Fig.6. According to the mean roughness (Ra and Rq) and maximum roughness (Rmax) in Table 2, the surfaces of nanohybrid membranes gradually change to be smoother with the addition of SiO2@GO compared to the neat membrane M-1. However, membrane M-5 deviates from this trend. It becomes much rougher with 1.2wt.% SiO2@GO in the dope, but still show smoother surface than that of neat membrane M-1. The roughness difference should be contributed to the nucleation and growth of polymer induced by the addition of SiO2@GO. Up to the 0.9wt.%(membrane M-4), the added SiO2@GO nano fillers induce much more nucleuses and smaller crystal grains, which leads to the smoother top surface. However, when SiO2@GO content attains 1.2wt.% (membrane M-5), the viscosity of cast solution dominates the phase separation process[35]. From Table 2, the results of AFM images of membranes are consistent with the FESEM images in Fig.2. 3.3 The hydrophilicity and static protein adsorption of PVDF/SiO2@GO hybrid membranes
110 105 100
Contact angle()
95 90 85 80 75 70
M-1 M-2 M-3 M-4 M-5
65 60 55 50 45 0
100
200
300
400
500
600
Time(s) Fig.7 The dynamic contact angles of PVDF/SiO2@GO nanohybrid membranes
12
The surface hydrophilicity of PVDF/SiO2@GO hybrid membranes is detected by the dynamic contact angle of water drop. As shown in Fig.7, the surface contact angle evidently decreases with the loading of SiO2@GO in the membranes, indicating the increase of surface hydrophilicity[36]. Therefore, the nanohybrid membranes with SiO2@GO loading should get higher antifouling ability [36-38]. From Fig.7, it also can be seen that the surface contact angle of M-5 decrease much more quickly than other samples. It may be result of the much larger pores on the surface, which promotes the penetration of water drop into membrane matrix.
138.7
137.1
140
M-1
M-2
108.6
2
BSA absorption capacity(g/cm )
160
120
74.7
100
62.4
80
60
M-3
M-4
M-5
Fig.8 The static absorption capacities of PVDF/SiO2@GO nanohybrid membranes for BSA aqueous solution. To investigate the protein resistance, the static absorption properties of PVDF/SiO2@GO nanohybrid membranes for BSA aqueous solution are carried out. The results are illustrated in Fig.8. It can be seen that the static absorption capacity of nanohybrid membranes for BSA protein clearly decreases with the loading of SiO2@GO. A BSA adsorption capacity of 138.7±1.2 μg/cm2 is obtained for the bare PVDF membrane (M-1), which gradually decreases to 74.7±0.5 μg/cm2 and 62.4±1.2 μg/cm2 for membrane M-4 and membrane M-5 respectively. Obviously the nanohybrid membranes containing more SiO2@GO exhibit lower absorption capacity, which will improve the protein fouling ability of membranes in filtration process. The variation of static absorption for BSA is well accordance with the hydrophilicity of PVDF/SiO2@GO hybrid membranes. The previous work[10] [39, 40]also demonstrated that hydrophilicity enhancement can effectively mitigate the
13
protein adsorption of membranes. 3.4 The permeation performances of PVDF/SiO2@GO nanohybrid membranes Table 3 The flux(JW1), BSA rejection(R)，melting temperature(Tm) and crystallinity(xc) of PVDF/SiO2@GO nanohybrid membranes Membrane no.
JW1(L·m-2·h-1·bar-1)
R (%)
Tm(°C)
xc(%)
M-1 M-2 M-3 M-4 M-5
268.5±10.5 220.1±9.4 192.0±8.4 182.6±5.6 679.1±20.5
63.5±0.3 72.6±0.2 83.8±0.2 91.7±0.4 34.6±1.2
162.9 163.4 164.0 164.6 163.3
44.8 45.4 47.2 47.6 45.2
Solute separation and solvent permeation, as two essential properties of membranes, are estimated by BSA rejection and pure water permeation flux. It is well known that the BSA rejection and water flux are correlated with the pore size, porosity, surface properties and membrane structure. As shown in Table 3, compared to the bare PVDF membrane M-1, the pure water flux of nanohybrid membranes slowly falls off, and the BSA rejection evidently increases to 91.7% with the addition of SiO2@GO up to 0.9wt.% in the dopes. Nevertheless, compared with other membranes, the flux of membrane M-5 markedly increases about three times and attains 679.1 L·m-2·h-1·bar-1 accompanied a much lower BSA rejection. Although many factors including the mean pore size, porosity and surface hydrophilicity influence the permeability of membranes, permeation theory predicts that the filtration rate is directly proportional to the square of the effective pore size of the membrane. The nanohybrid membranes are demonstrated with the smaller pore size, denser top surface and thus the lower permeation flux despite they shows better hydrophilicity with the loading of SiO2@GO. Obviously, the flux improvement of membrane M-5 is attributed to its larger pore size and better hydrophilicity. What’s more, according to the water and the BSA rejection, the M-4 and M-5 show the typical ultrafiltration and microfiltration performances respectively. For the pressure-driven filtration membranes, such as ultrafiltration which only allows solute with certain size to pass through effective pore of membrane surface, size-sieving is primary factor influencing on the solute separation[10, 41]. It is really amazed that the large increase in permeability and decrease in BSA rejection from membrane M-4 to membrane M-5, which will be further investigated. Nevertheless, the overhigh viscosity of cast dope may be one of the reasons, which mainly due to the GO@SiO2 additive increases and blocks the movement of polymer chains, bring the higher porosity and bigger surface pore size. Above disscusion verifies that SiO2@GO addition evidently influences the permeation performances of PVDF/SiO2@GO nanohybrid membranes. However, from Fig.S2,
14
prinstine GO(without decoration of silica) addition has little effects on the permeation flux and BSA rejection of PVDF/GO hybrid membranes, which is accordance with the top morphology. 3.5 Long- term stability test of PVDF/SiO2@GO nanohybrid membranes Although polymeric membranes gain wide acceptance due to the simple operation, low cost and acceptable discharge quality, they are usually faced with the serious fouling and flux decline due to absorption and/or pore plugging, which leads to the productivity decline and increase of operation cost[42, 43]. In order to clearly investigate long-term stability test of PVDF/SiO2@GO hybrid membranes, two cycles of dynamic filtration test are addressed below. The membrane fluxes as a function of time are illustrated in Fig.9. During the BSA filtration process, the permeation flux of protein solution declines and then gets stable after 30 min filtration, which is resulted from the protein adsorption on membrane surface, pore plugging and concentration polarization etc. After washing with NaClO aqueous solution for 30min, the pure water flux rebounds in different degrees. To quantitatively evaluate the anti-fouling property of membranes, the reversible fouling ratio (Rr), irreversible fouling ratio (Rir) and total fouling ratio (Rt) and the flux recovery ratio (FRR) parameters of each membrane are calculated and presented in Fig. 10. It can be seen that the Rt and Rir of bare PVDF membrane M-1 are 72.9% and 52.5%, respectively. However, the nanohybrid membranes present a falling tendency in both Rt and Rir. Especially for membrane M-5, the Rir value even declines to 3.4%, which means the water flux can almost be recovered completely. Moreover, the FRR values of the nanohybrid membranes increase with the addition of SiO2@GO nanosheets despite of the membrane surface structure, which is positively correlation with the surface hydrophilicity of membranes. Depending on the hydrophilicity of membrane surface and the results of dynamic filtration, it can be concluded that the addition of SiO2@GO nanohybrid in the membranes enhances the antifouling performances.
15
700
M-1 M-2 M-3 M-4 M-5
Jw1 600
-1
Flux(Lm h bar )
500
-2
-1
400
300
200
100
BSA
0 0
NaClO
20
NaClO
BSA
40
60
80
100
120
Time(min)
Fig. 9 Time-dependent fluxes of PVDF/SiO2@GO nanohybrid membranes (M-1~M-5)
100
FRR(%)
Rt(%)
Rr(%)
Rir(%)
Percentage(%)
80
60
40
20
0 M-1
M-2
M-3
M-4
M-5
Fig.10 The fouling parameters of PVDF/SiO2@GO nanohybrid membranes
16
3.6 The crystallization behaviors of PVDF/SiO2@GO nanohybrid membranes In general, PVDF chains can crystallize into at least three distinct forms: α, β and γ crystalline phases. In order to measure the crystallation properties of as-prepared membranes, WXRD pattern is applied to inspect the PVDF/SiO2@GO nanohybrid membranes. Four striking diffraction peaks appear at 17.5°,18.4°, 19.9° and 26.5°, which correspond to the reflections of (100), (020), (110), (021) planes of α-phase with a trans-gauche (TGTG) PVDF crystals[44]. Generally, high temperature favors the formation of α-phase and low temperature induces β-phase [9, 45]. The WXRD patterns of PVDF/SiO2@GO nanohybrid membranes shown in Fig. S3 well accord with this regularity. Furthermore, despite its very low content of SiO2@GO in the membranes, the newly obtuse peak at about 23~24° demonstrates the existence of amorphous silica coating [31, 46], which is
764
795
872 855 840
974
the apparent evidence of SiO2@GO.
M-1
Transmittance(a.u.)
M-2 M-3 M-4 M-5 Si-OH
1000 -1
Wavenumber(cm )
Fig. 11 The ATR-FTIR spectra of PVDF/SiO2@GO nanohybrid membranes To further examine the polymorphism of PVDF, the ATR-FTIR spectra of the hybrid membranes were performed. As shown in Fig.12, the bands at 974, 872, 855, 795 and 764 cm-1 are the characteristic bands of α-phase PVDF [1, 44, 47]. Once SiO2@GO is introduced into the matrix, a weak band appears at 840 cm-1 for the nanohybrid membranes (M-2~M-5), which is ascribed to β-phase of PVDF [4, 44, 48]. From Fig. 12, compared to the neat membrane of M-1, the spectra of the nanohybrid membranes show a weak band at
17
950cm-1 (νSi-OH), which ascertains that SiO2@GO is successfully integrated into the hybrid membranes. The DSC curves of PVDF/SiO2@GO nannohybrid membranes with different additive contents are presented in Fig. S4, and the melting temperature(Tm) and crystallinity(xc) of membranes are listed in Table 3. Compared to the neat membrane M-1, the melting temperature and crystallinity of hybrid membranes initially increase with the content of SiO2@GO up to 0.9wt.% for membrane M-4, and then decline as the SiO2@GO concentration is further increased. These behaviors can be interpreted that SiO2@GO nano fillers in the matrix results in the combined effect of nucleation and growth of PVDF crystal and viscosity increase of cast solution. The appropriate amount of additive can prompt the PVDF crystallization; at the same time the viscosity increase of cast solution inhibits crystal growth on the contrary. Consequently, the membrane crystallization trend agrees with hypothesis of membrane mechanism above. 4. Conclusions The derivative of GO, SiO2@GO nanohybrid, is fabricated and employed to synthesize PVDF/SiO2@GO nanohybrid membranes via TIPS method for the first time. The following conclusions can be drawn: (1) The synthesized PVDF/SiO2@GO nanohybrid membranes experience liquid-liquid phase separation mechanism, and exhibit bi-continual and asymmetric structure as well as outstanding fibrous top surface and sheaf-like bottom surface. The included SiO2@GO nanohybrid is uniformly dispersed in the membrane matrix. With the addition of SiO2@GO, the top surface becomes denser and the pore size decreases; but overhigh SiO2@GO addition for M-5 triggers in the adverse trend. This is caused by the combined actions of nucleation and growth of PVDF and viscosity increase of cast solution due to the addition SiO2@GO nanohybrid. (2) The BSA rejection of membranes gradually increases with the addition of PVDF/SiO2@GO nanohybrid accompanied with the decline of pure water permeation flux. As the SiO2@GO content increases to 0.9wt.%, the PVDF/SiO2@GO nanohybrid membrane M-4 presents the highest BSA rejection of 91.7% and the lowest permeation flux of 182.6 L·m-2·h-1·bar-1. However, the overhigh SiO2@GO addition (1.2wt.%) leads to the outstanding pure water permeation flux of 679.1 L·m-2·h-1·bar-1 and a much lower BSA rejection. The permeation flux and BSA rejection are dominatedly related with the pore size despite the variations of other parameters. (3) With the addition of PVDF/SiO2@GO nanohybrid, the resultant membranes exhibit a lower BSA adsorption tend, an improved surface hydrophilicity and antifouling ability. (4) The XRD patterns and FTIR spectra of membranes exhibit the exclusive α-phase of PVDF, and the 18
melting temperature(Tm) and crystallinity(xc) evidently increase with the addition of PVDF/SiO2@GO nanohybrid up to 0.9wt.% in the dopes. Overhigh SiO2@GO addition (1.2wt.%) leads to small decline for these parameters.
Acknowledgements The research is supported by Science and Technology Commission of Shanghai Municipality (13ZR1429900, 14520502900) and International Joint Laboratory on Resource Chemistry (IJLRC).
Nomenclature JW1 pure water permeability (L·m-2·h-1·bar-1). Jw2 pure water permeability after washing with NaClO solution (L·m-2·h-1·bar-1). Jp1 The BSA aqeous permeability(L·m-2·h-1·bar-1). R
the rejection for solute (%).
V
the volume of pure water (L).
∆P the transmembrane pressure (bar/MPa)). A
the effective membrane area (m-2).
CP the concentrations of permeate (g/L). CF the concentrations of feed solutions (g/L). FRR
the flux recovery ratio (%).
Rt
total fouling ratio (%)
Rr
the reversible fouling ratio(%).
Rir the irreversible fouling ratio (%). xc ∆Hf
the crystallinity of membranes(%) The melting enthalpy of membranes (J/g)
∆Hf* The melting enthalpy of the neat PVDF(104.7J/g) Tm The melting temperature of samples(°C)
References [1] F. Liu, N.A. Hashim, Y. Liu, M.R.M. Abed, K. Li, Progress in the production and modification of PVDF membranes, Journal of Membrane Science, 375 (2011) 1-27. [2] X. Huang, W. Wang, Y. Liu, H. Wang, Z. Zhang, W. Fan, L. Li, Treatment of oily waste water by PVP 19
grafted PVDF ultrafiltration membranes, Chemical Engineering Journal, 273 (2015) 421-429. [3] G.-d. Kang, Y.-m. Cao, Application and modification of poly(vinylidene fluoride) (PVDF) membranes – A review, Journal of Membrane Science, 463 (2014) 145-165. [4] C.Y. Lai, A. Groth, S. Gray, M. Duke, Impact of casting conditions on PVDF/nanoclay nanocomposite membrane properties, Chemical Engineering Journal, 267 (2015) 73-85. [5] S. Berghmans, H. Berghmans, H.E.H. Meijer, Spinning of hollow porous fibres via the TIPS mechanism, Journal of Membrane Science, 116 (1996) 171-189. [6] J.A. de Lima, M.I. Felisberti, Porous polymer structures obtained via the TIPS process from EVOH/PMMA/DMF solutions, Journal of Membrane Science, 344 (2009) 237-243. [7] Z. Yang, P. Li, L. Xie, Z. Wang, S.-C. Wang, Preparation of iPP hollow-fiber microporous membranes via thermally induced phase separation with co-solvents of DBP and DOP, Desalination, 192 (2006) 168-181. [8] Z. Cui, N.T. Hassankiadeh, S.Y. Lee, J.M. Lee, K.T. Woo, A. Sanguineti, V. Arcella, Y.M. Lee, E. Drioli, Poly(vinylidene fluoride) membrane preparation with an environmental diluent via thermally induced phase separation, Journal of Membrane Science, 444 (2013) 223-236. [9] S.-i. Sawada, C. Ursino, F. Galiano, S. Simone, E. Drioli, A. Figoli, Effect of citrate-based non-toxic solvents on poly(vinylidene fluoride) membrane preparation via thermally induced phase separation, Journal of Membrane Science, 493 (2015) 232-242. [10] H.-P. Xu, W.-Z. Lang, X. Yan, X. Zhang, Y.-J. Guo, Preparation and characterizations of poly(vinylidene fluoride)/oxidized multi-wall carbon nanotube membranes with bi-continuous structure by thermally induced phase separation method, Journal of Membrane Science, 467 (2014) 142-152. [11] A. Cui, Z. Liu, C. Xiao, Y. Zhang, Effect of micro-sized SiO2-particle on the performance of PVDF blend membranes via TIPS, Journal of Membrane Science, 360 (2010) 259-264. [12] P.M. Atkinson, D.R. Lloyd, Anisotropic flat sheet membrane formation via TIPS: thermal effects, Journal of Membrane Science, 171 (2000) 1-18. [13] P.M. Atkinson, D.R. Lloyd, Anisotropic flat sheet membrane formation via TIPS: atmospheric convection and polymer molecular weight effects, Journal of Membrane Science, 175 (2000) 225-238. [14] G.-L. Ji, L.-P. Zhu, B.-K. Zhu, C.-F. Zhang, Y.-Y. Xu, Structure formation and characterization of PVDF hollow fiber membrane prepared via TIPS with diluent mixture, Journal of Membrane Science, 319 (2008) 264-270. [15] N.T. Hassankiadeh, Z. Cui, J.H. Kim, D.W. Shin, A. Sanguineti, V. Arcella, Y.M. Lee, E. Drioli, PVDF hollow fiber membranes prepared from green diluent via thermally induced phase separation: Effect of PVDF molecular weight, Journal of Membrane Science, 471 (2014) 237-246. [16] R. Carr, I.A. Weinstock, A. Sivaprasadarao, A. Muller, A. Aksimentiev, Poly(vinylidene fluoride) membrane preparation with an environmental diluent via thermally induced phase separation, Journal of Membrane Science, 8 (2008) 3916. [17] Y.-R. Qiu, H. Matsuyama, Preparation and characterization of poly(vinyl butyral) hollow fiber membrane via thermally induced phase separation with diluent polyethylene glycol 200, Desalination, 257 (2010) 117-123. [18] T. Xiao, P. Wang, X. Yang, X. Cai, J. Lu, Fabrication and characterization of novel asymmetric polyvinylidene fluoride (PVDF) membranes by the nonsolvent thermally induced phase separation (NTIPS) method for membrane distillation applications, Journal of Membrane Science, 489 (2015) 160-174. [19] X. Li, X. Lu, Morphology of polyvinylidene fluoride and its blend in thermally induced phase separation process, Journal of Applied Polymer Science, 101 (2006) 2944-2952. [20] H.B. Yuxi Xu, Gewu Lu, Chun Li, and Gaoquan Shi, Flexible Graphene Films via the Filtration of 20
Water-Soluble Noncovalent Functionalized Graphene Sheets, Journal of the American Chemical Society, 130 (2008) 5856-5857. [21] H. Li, S. Pang, S. Wu, X. Feng, K. Mullen, C. Bubeck, Layer-by-layer assembly and UV photoreduction of graphene-polyoxometalate composite films for electronics, Journal of the American Chemical Society, 133 (2011) 9423-9429. [22] S. Lee, B.G. Choi, D. Choi, H.S. Park, Nanoindentation of annealed Nafion/sulfonated graphene oxide nanocomposite membranes for the measurement of mechanical properties, Journal of Membrane Science, 451 (2014) 40-45. [23] J. Lee, H.-R. Chae, Y.J. Won, K. Lee, C.-H. Lee, H.H. Lee, I.-C. Kim, J.-m. Lee, Graphene oxide nanoplatelets composite membrane with hydrophilic and antifouling properties for wastewater treatment, Journal of Membrane Science, 448 (2013) 223-230. [24] H. Wu, B. Tang, P. Wu, Development of novel SiO2–GO nanohybrid/polysulfone membrane with enhanced performance, Journal of Membrane Science, 451 (2014) 94-102. [25] N. Wang, S. Ji, J. Li, R. Zhang, G. Zhang, Poly(vinyl alcohol)–graphene oxide nanohybrid “pore-filling” membrane for pervaporation of toluene/n-heptane mixtures, Journal of Membrane Science, 455 (2014) 113-120. [26] Z. Xu, J. Zhang, M. Shan, Y. Li, B. Li, J. Niu, B. Zhou, X. Qian, Organosilane-functionalized graphene oxide for enhanced antifouling and mechanical properties of polyvinylidene fluoride ultrafiltration membranes, Journal of Membrane Science, 458 (2014) 1-13. [27] Y.P. Tang, D.R. Paul, T.S. Chung, Free-standing graphene oxide thin films assembled by a pressurized ultrafiltration method for dehydration of ethanol, Journal of Membrane Science, 458 (2014) 199-208. [28] H.M. Hegab, L. Zou, Graphene oxide-assisted membranes: Fabrication and potential applications in desalination and water purification, Journal of Membrane Science, 484 (2015) 95-106. [29] W.S. Hummers, R.E. Offeman, Preparation of Graphitic Oxide, Journal of the American Chemical Society, 80 (1958) 1339-1339. [30] D.C. Marcano, D.V. Kosynkin, J.M. Berlin, A. Sinitskii, Z. Sun, A. Slesarev, L.B. Alemany, W. Lu, J.M. Tour, Improved synthesis of graphene oxide, ACS Nano, 4 (2010) 4806-4814. [31] L. Kou, C. Gao, Making silica nanoparticle-covered graphene oxide nanohybrids as general building blocks for large-area superhydrophilic coatings, Nanoscale, 3 (2011) 519-528. [32] X. Li, H. Liu, C. Xiao, S. Ma, X. Zhao, Effect of take-up speed on polyvinylidene fluoride hollow fiber membrane in a thermally induced phase separation process, Journal of Applied Polymer Science, 128 (2013) 1054-1060. [33] Y. Shen, A.C. Lua, Preparation and characterization of mixed matrix membranes based on PVDF and three inorganic fillers (fumed nonporous silica, zeolite 4A and mesoporous MCM-41) for gas separation, Chemical Engineering Journal, 192 (2012) 201-210. [34] K.G. Lee, R. Wi, M. Imran, T.J. Park, J. Lee, S.Y. Lee, D.H. Kim, Functionalization Effects of Single-Walled Carbon Nanotubes as Templates for the Synthesis of Silica Nanorods and Study of Growing Mechanism of Silica, ACS Nano, 4 (2010) 3933-3942. [35] J. Wang, W.-Z. Lang, H.-P. Xu, X. Zhang, Y.-J. Guo, Improved poly(vinyl butyral) hollow fiber membranes by embedding multi-walled carbon nanotube for the ultrafiltrations of bovine serum albumin and humic acid, Chemical Engineering Journal, 260 (2015) 90-98. [36] X. Zhang, W.-Z. Lang, X. Yan, Z.-Y. Lou, X.-F. Chen, Influences of the structure parameters of multi-walled carbon nanotubes(MWNTs) on PVDF/PFSA/O-MWNTs hollow fiber ultrafiltration membranes, Journal of Membrane Science, 499 (2016) 179-190. 21
[37] X. Li, M. Wang, C. Wang, C. Cheng, X. Wang, Facile Immobilization of Ag Nanocluster on Nanofibrous Membrane for Oil/Water Separation, ACS Applied Materials & Interfaces, 6 (2014) 15272-15282. [38] Y. Han, Y. Jiang, C. Gao, High-Flux Graphene Oxide Nanofiltration Membrane Intercalated by Carbon Nanotubes, ACS Applied Materials & Interfaces, 7 (2015) 8147-8155. [39] W.-Z. Lang, J.-P. Shen, Y.-X. Zhang, Y.-H. Yu, Y.-J. Guo, C.-X. Liu, Preparation and characterizations of charged poly(vinyl butyral) hollow fiber ultrafiltration membranes with perfluorosulfonic acid as additive, Journal of Membrane Science, 430 (2013) 1-10. [40] W.-Z. Lang, Q. Ji, J.-P. Shen, Y.-J. Guo, Z.-L. Xu, The roles of alkali metal counter-ions of PFSA play in the formation of PVDF/PFSA-M hollow fiber membranes, Desalination, 292 (2012) 45-52. [41] Z. Shi, W. Zhang, F. Zhang, X. Liu, D. Wang, J. Jin, L. Jiang, Ultrafast separation of emulsified oil/water mixtures by ultrathin free-standing single-walled carbon nanotube network films, Advanced materials, 25 (2013) 2422-2427. [42] S. Zhang, R. Wang, S. Zhang, G. Li, Y. Zhang, Development of phosphorylated silica nanotubes (PSNTs)/polyvinylidene fluoride (PVDF) composite membranes for wastewater treatment, Chemical Engineering Journal, 230 (2013) 260-271. [43] Y. Zhang, L. Wang, Y. Xu, ZrO2 solid superacid porous shell/void/TiO2 core particles (ZVT)/polyvinylidene fluoride (PVDF) composite membranes with anti-fouling performance for sewage treatment, Chemical Engineering Journal, 260 (2015) 258-268. [44] P. Martins, A.C. Lopes, S. Lanceros-Mendez, Electroactive phases of poly(vinylidene fluoride): Determination, processing and applications, Progress in Polymer Science, 39 (2014) 683-706. [45] I.J. Roh, S. Ramaswamy, W.B. Krantz, A.R. Greenberg, Poly(ethylene chlorotrifluoroethylene) membrane formation via thermally induced phase separation (TIPS), Journal of Membrane Science, 362 (2010) 211-220. [46] M. Obaid, G.M.K. Tolba, M. Motlak, O.A. Fadali, K.A. Khalil, A.A. Almajid, B. Kim, N.A.M. Barakat, Effective polysulfone-amorphous SiO2 NPs electrospun nanofiber membrane for high flux oil/water separation, Chemical Engineering Journal, 279 (2015) 631-638. [47] T. Boccaccio, A. Bottino, G. Capannelli, P. Piaggio, Characterization of PVDF membranes by vibrational spectroscopy, Journal of Membrane Science, 210 (2002) 315-329. [48] J. Wang, H. Li, J. Liu, Y. Duan, S. Jiang, S. Yan, On the α → β Transition of Carbon-Coated Highly Oriented PVDF Ultrathin Film Induced by Melt Recrystallization, Journal of the American Chemical Society, 125 (2003) 1496-1497.
Highlights:
SiO2@GO nanohybrids with high thermal stability are synthesized.
PVDF/SiO2@GO nanohybrid membranes are prepared via TIPS method.
SiO2@GO addition leads to permeation flux decline and BSA rejection increasing.
SiO2@GO addition improves the hydrophilicity and anti-fouling ability of membranes.
22
PII: DOI: Reference:
S0376-7388(16)30186-7 http://dx.doi.org/10.1016/j.memsci.2016.03.048 MEMSCI14386
To appear in: Journal of Membrane Science Received date: 2 December 2015 Revised date: 9 March 2016 Accepted date: 23 March 2016 Cite this article as: Zhong-Kun Li, Wan-Zhong Lang, Wei Miao, Xi Yan and YaJun Guo, Preparation and properties of PVDF/SiO2@GO nanohybrid membranes via thermally induced phase separation method, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2016.03.048 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Preparation and properties of PVDF/SiO2@GO nanohybrid membranes via thermally induced phase separation method Zhong-Kun Li, Wan-Zhong Lang*, Wei Miao, Xi Yan, Ya-Jun Guo The Education Ministry Key Laboratory of Resource Chemistry and Shanghai Key Laboratory of Rare Earth Functional Materials, Department of Chemistry and Chemical Engineering, Shanghai Normal University, 100 Guilin Road, Shanghai 200234, China. *
Corresponding author. Tel: +86 21 64321951; fax: +86 21 64321951. [email protected]
Abstract
Graphene oxide (GO), one of the most promising filter materials, is intensively employed to prepare the membranes via non-solvent induced phase separation(NIPS) method, but it hasn’t been found to modify polymeric membranes via thermally induced phase separation (TIPS) method. In this work, the derivative of GO, SiO2@GO nanohybrid, is fabricated and employed to synthesize PVDF/SiO2@GO nanohybrid membranes via TIPS method for the first time. The results indicate that the PVDF/SiO2@GO nanohybrid membranes experience liquid-liquid phase separation mechanism, and exhibit bi-continual and asymmetric structure. The included SiO2@GO nanohybrid is uniformly dispersed in the membrane matrix. With the addition of SiO2@GO, the top surface becomes denser and the pore size decreases; but overhigh SiO2@GO addition for membrane M-5 triggers in the adverse trend. This is caused by the combined actions of nucleation and growth of PVDF and viscosity increase of cast solution due to the addition SiO2@GO nanohybrid. The BSA rejection of membrane gradually increases with the addition of PVDF/SiO2@GO nanohybrid accompanied with the decline of pure water permeation flux. As the SiO2@GO content increases to 0.9wt.%, the PVDF/SiO2@GO nanohybrid membrane M-4 presents the highest BSA rejection of 91.7% and the lowest permeation flux of 182.6 L·m-2·h-1·bar-1. However, the overhigh SiO2@GO addition (1.2wt.%) leads to the outstanding pure water permeation flux of 679.1 L·m-2·h-1·bar-1 and a much lower BSA rejection. The addition of PVDF/SiO2@GO nanohybrid evidently improves the surface hydrophilicity and antifouling ability of resultant membranes. The XRD patterns and FTIR spectra of membranes verify the exclusive α-phase of PVDF, and the melting temperature(Tm) and crystallinity(xc) evidently increase with the addition of SiO2@GO nanohybrid up to 0.9wt.% in the dopes. Overhigh SiO2@GO addition (1.2wt.%) leads to small decline for 1
these parameters. Graphical Abstract Keywords: poly(vinylidene fluoride) (PVDF); graphene oxide (GO); ultrafiltration; thermally induced phase separation(TIPS); SiO2@GO nanohybrid
1. Introduction Membrane separation technology is widely used to remove various water contaminants, which have significant adverse health and/or environmental effects [1, 2]. Due to the outstanding properties such as high thermal stability, good chemical resistance, high mechanical strength, good aging resistance and membrane forming properties, poly(vinylidene fluoride)(PVDF) membranes were applied to a range of scientific researches and industrial processes[3, 4]. To date, most commercial and lab membranes are produced via a phase inversion process. Thermally induced phase separation (TIPS) method was recognized since the late 1980s; and from then on much more works were devoted to prepare porous membranes via TIPS method[5, 6]. Apart from the polymers such as polypropylene (PP)[7] and polyethylene (PE), the PVDF membranes[8-11] prepared by TIPS methods were found more significant in mechanical strength, pore size distribution and permeation performances compared with those produced by non-solvent induced phase separation(NIPS) method. In the past years, most works in membrane preparations via TIPS process were intensively concentrated on the selection and effects of diluents on the membrane structure and permeation performance [9, 12-18]. For instance, Atkinson et al.[12, 13] employed isotactic polypropylene (iPP) and diphenyl ether (DPE) to produce flat membranes with an anisotropic structure via TIPS, which underwent a liquid-liquid phase separation mechanism. Ji et al.[14] selected dibutyl phthalate (DBP) and di(2-ethylhexyl) phthalate (DEHP) as diluents to prepare PVDF hollow fiber membranes. In fact, the additives also markedly influence the membrane structure and performance in TIPS process, which is similar to NIPS process. Nevertheless, only a few works were paid on this side. For example, Li et al.[19] reported that CaCO3 influenced the crystallization kinetics of ternary mixture of PVDF/CaCO3/DBP in preparing membranes via TIPS. Cui et al.[11] found that a certain amount of micro-sized SiO2 promoted the water flux of PVDF/SiO2 blend membranes and induced the formation of spherulitic morphology. Xu et al.[10] fabricated the PVDF/oxidized multi-wall carbon nanotubes(O-MWNTs) flat membranes with bi-continuous 2
structures. Graphene and its derivatives(such as graphene oxide, GO) are considered as one of the most promising filter materials[20, 21]. Although they were intensively employed to prepare the membranes with high anti-fouling and rejection performances via non-solvent induced phase separation (NIPS) method [22-28], the graphene and its derivatives (such as GO) have not been found to modify polymeric membranes via TIPS method. The low thermal stability of GO may be one factor inhibiting its applications in TIPS process. In order to overcome the drawback of graphene and its derivatives, SiO2@GO nanohybrid was produced via in situ hydrolysis and condensation of TEOS on the surfaces of graphene oxide (GO) nanosheets, and then used to decorate PVDF membranes via TIPS. This study aims to synthesize PVDF/SiO2@GO nanohybrid membranes for the first time. The influences of SiO2@GO nanohybrid on the membrane structure and permeation performances were detailedly investigated upon a series of characterizations. 2. Experimental 2.1 Materials Poly(vinylidene fluoride) (PVDF, FR904) was purchased from Shanghai 3F New Material Co. Ltd. (China). Graphite powder (325 mesh) and tetraethoxysilane (TEOS, 98%) were purchased from Aladdin Chemistry Co. Ltd. (Shanghai, China). Ammonia solution (NH3·H2O, 25wt%), dibutyl phthalate (DBP), hydrochloric acid( HCl, 36~38wt%), H2SO4(98wt%) and absolute ethanol were provided by Shanghai Chemical Agent Co. Ltd.(China). Deionized water (DI) was produced by a reverse osmosis (RO) system. Bovine serum albumin (BSA, MW=67000) was purchased from Shanghai Bio Co. Ltd. (China). All chemicals were used without further purification. 2.2 Preparation of inorganic particles 2.2.1 Synthesis of GO nanosheets Graphene oxide was synthesized from oxidized natural graphite flakes by a modified Hummers method [29, 30]. In a typical procedure, 3.0g graphite flakes and 18.0g KMnO4 were added to a mixed H2SO4/H3PO4 (volume ratio=360:40) solution; then the mixture was stirred and reacted at 50 °C for 12h. After the reaction, the mixtures were cooled to room temperature and poured into the ice bath with adding 3ml 30% H2O2, and then the remaining solid material was washed with DI water, 30% HCl aqueous solution, and absolute ethanol in succession. The solid was dried in vacuum oven at room temperature and stored for further study. 2.2.2 Synthesis of SiO2@GO nanohybrid Silica nanoparticles were deposited on GO nanosheets by in situ hydrolysis and condensation of 3
TEOS[31]. In a typical procedure, 0.5 g GO was dispersed in 420 ml alcohol–water (5/1, v/v) solution by sonication for 1 h. After that, the pH of reaction mixture was adjusted to approximate 9.0 with ammonia solution, and then 5.5 mL TEOS was added into the solution drop by drop. After being vigorously dispersed in ultrasonic bath for 1.5 h, the mixture was stood for 24 h at room temperature (25 °C). Then, the SiO2@GO suspension was centrifuged, washed for 5 times with alcohol, dried in vacuum. The obtained SiO2@GO nanohybrid was stored for further study. 2.2.3 Characterizations of GO and SiO2@GO The synthesized GO nanosheets and GO-SiO2 nanohydrids were detected by Fourier-transform infrared spectroscopy (FTIR Electron Corp Nicolet 380, US) in the range of 4000–400 cm−1 at room temperature. The thermal stability of GO nanosheets and SiO2@GO nanohybrid was measured by a thermal gravimetric analyzer (Perkin-Elmer Pyris 6 TGA instrument) at a heating rate of 10°C/min from 35°C to 800°C under nitrogen atmosphere. The morphologies of GO nanosheets and SiO2@GO nanohybrid were detected with a field emission transmission electron microscope (FETEM, JEM-2010, Japan) at 200 kV. 2.3 Membranes 2.3.1 Membrane preparation The PVDF/SiO2@GO nanohybrid membranes were prepared via TIPS process. The predetermined amount of SiO2@GO nanohybrid was completely dispersed in the measured amount of DBP diluent under ultrasonic bath for 1 h. Then 25 g of PVDF powder was added into the mixing solution. The homogenous casting solution was obtained by vigorous stirring at 200 °C for 6 h. After fully degassing, the casting solution was uniformly spread onto a heated stainless steel plate (200°C) with a knife gap of 450μm, and then immediately immersed into quenching bath (ice water bath) until totally solidification. To eliminate diluent, the nascent membranes were immersed into ethanol liquid for 12 h, and then soaked in DI water to extract the residual ethanol. The resulted membranes were then stored in DI water for further study in details. The detailed preparation
parameters for the membranes are were listed in Table 1. The PVDF and SiO2@GO contents in Table 1 represent the ones in the casting solutions. Table 1 Compositions of dope solutions and preparation temperature for PVDF/SiO2@GO membranes Mass ratio Membrane no
PVDF(g)
DBP(g)
SiO2@GO(g)
4
Membrane preparation temperature (°C)
Quenching temperature (°C)
M-1 M-2 M-3 M-4 M-5
25 25 25 25 25
75 75 75 75 75
0 0.3 0.6 0.9 1.2
200 200 200 200 200
0 0 0 0 0
2.3.2 Characterizations of membranes The membrane surfaces were analyzed by the attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) (Thermo Electron Corp., Nicolet 380, USA). The morphologies of the as-prepared membranes were detected using a field emission scanning electron microscopy (FESEM, Hitachi S-4800, Japan) equipped with an energy-dispersive spectrometry (EDS) under an accelerating voltage of 5.0 kV. Before recording the cross-section SEM images, the membrane samples were fractured in a liquid nitrogen and then sputtered with gold for 120 seconds. The AFM images of top surfaces were performed on an atomic-force microscopy (AFM, BioScope TM, USA) with tapping mode. The surface was imaged at a scan size of 2 μm×2 μm at a speed of 2 Hz. The hydrophilicity of hybrid membrane was evaluated by water contact angle system (KRUSS DSA30 German) equipped with video at room temperature. The contact angle change of 3.0 μL water drop on the top surface was recorded in air. The crystalline properties of membranes were investigated by a wide-angle X-ray diffraction (WAXD, D/max-II B, Japan) in the 2θ range of 5–55°. The crystallinity and thermal properties of membranes were measured using a DSC (DSCQ100 TA Instruments USA) measurement in N2 atmosphere between 25°C and 280°C. The melting enthalpy (∆Hf) values of membranes were calculated according to the DSC curves. As a result, the crystallinity (xc) was calculated on the basis of the following equation (1):
xc
ΔH f ΔH *f
100%
(1)
where △Hf* is the melting enthalpy of the neat PVDF(104.7 J/g)[32, 33]. To investigate the protein adsorption, the membranes with a diameter of 7 cm were soaked in 1 g·L-1 BSA aqueous solution (PBS buffer solution as a solvent, pH=7) at room temperature for 24 h. The protein concentrations before and after adsorption were measured by an ultraviolet spectrophotometer (UV3600 Shimadzu, Japan) at 280 nm. The adsorption capacity of each membrane was calculated according to the concentration difference before and after adsorption. The pure water permeation flux (JW1) and BSA rejections of as-prepared membranes were carried out by a self-made lab device with a valid membrane area of 26.04 cm2. First, all samples were compressed at 2 bar 5
for 30min, then the JW1 values of membranes were measured at 25 °C at 1 bar using the following equation (2):
JW 1
V A t P
(2)
where V is the volume of penetrative water (L), A is the effective membrane area (m2), t is the running time (h), and P is the trans-membrane pressure (bar). Afterwards the 500 ppm BSA aqueous solution as the feed solution was forced to permeate the membrane for 30 min to assess the fouling performance. The concentrations of permeate and feed solutions were determined by an ultraviolet spectrophotometer (UV3600 Shimadzu, Japan) at 280 nm. The rejection of BSA (R, %) was defined using the following equation (3):
R (1
cP ) 100% cF
(3)
where CP and CF are the concentrations of protein in the permeate and feed solutions, respectively. The permeation flux for protein aqueous was recorded as Jp1. After the filtration of BSA solution, the membrane was washed with 500 ppm NaClO aqueous solution at 25 °C for 30 min. After being rinsed with DI water, the pure water flux of the washed membrane was re-measured again and recorded as Jw2. To analyze the antifouling property, several parameters were defined as followed. The flux recovery ratio (FRR) was calculated by equation (4):
FRR
JW 2 100% JW 1
(4)
The reversible fouling ratio (Rr), irreversible fouling ratio (Rir) and total fouling ratio (Rt) were described as following:
J P1 100% JW 1
(5)
J w2 J P1 100% JW 1
(6)
Rt 1
Rr
Rt Rr Ri r
(7)
3. Results and discussion 3.1 Characterizations of GO and SiO2@GO Fig. 1 (a) depicts the FTIR spectra of GO and SiO2@GO nanohybrid. The peak at 1720 cm-1, which is related to the C=O vibration of carboxylic groups of GO, vanishes in the spectra of SiO2@GO [24, 31]. It can 6
be ascribed to the conversion of C=O on GO surface to Si-O-C after the reaction with TEOS, which demonstrates the interconnection between GO and silica coating. The spectrum of SiO2@GO show two new peeks at 1090 cm-1 and 465 cm-1, which are attributed to the Si–O–Si asymmetric vibration and bending vibrations respectively[34]. Also, the peaks at 800 cm-1 and 950 cm-1 are ascribed to the Si-O-Si symmetric vibration[31] and the Si-OH stretching band respectively. The thermal stability of material is vital to the structure and performance for the membranes prepared by TIPS method. The TGA curves of GO and SiO2@GO are presented in Fig.1 (b). The TGA curve of GO shows a marked weight loss at ~200 °C, which is mainly attributed to the decomposition of oxygen-containing groups. This is the reason that the neat GO as additive isn’t suitably used for membrane preparation by TIPS method. Nevertheless, after coating with silica, the phenomenon of thermal weight loss at ~200 °C isn’t found, and the thermal stability of SiO2@GO nanohybrid is evidently improved. It demonstrates the feasibility of using SiO2@GO nanohybrid as membrane material in TIPS process. The TEM image in Fig.1 (c) illustrates the ultra-thin and silk-like GO nanosheets, which indicates the successful chemical exfoliation of graphite via the modified Hummer’s method in this work. The TEM image of SiO2@GO in Fig.1 (d) shows that silica coating uniformly covers on GO nanosheets, which demonstrates clearly in situ hydrolysis and condensation of TEOS on the GO nanosheets.
Fig.1 (a) FTIR spectra of the GO and SiO2@GO; (b)TGA curves of the GO and SiO2@GO; (c)TEM image of 7
GO nanosheets; (d)TEM image of SiO2@GO nanosheets. 3.2 The morphologies of PVDF/SiO2@GO nanohybrid membranes From Figs.2-3, the top surfaces and bottom surfaces of PVDF/SiO2@GO nanohybrid membranes exhibit the outstanding fibrous and sheaf-like structures respectively, which are the characteristic morphology induced by the mechanism of liquid-liquid phase separation[10]. The top surfaces of membranes turn to denser and pore size becomes smaller with the loading of SiO2@GO until 0.9wt.% loading in the dope. However, the addition of SiO2@GO further increases to 1.2wt.% in the dope for membrane M-5, the top surface grows loser and pore size become larger. These phenomena may be explained that the introduction of SiO2@GO plays a typical role of nucleus and promotes the crystallization of PVDF. The crystallization process induces the different morphologies and pore sizes of the top surfaces. However, from Fig. S1, the addition of GO has little effect on the structure of PVDF/GO hybrid membranes, which may be due to the decomposition of GO sheets at high temperature in TIPS process. However, the bottom surfaces are evidently looser than the top surfaces, which may be ascribed to the combined effects of two sides: the loss of diluent near the top surface and the slower cooling rate near the bottom surface during the membrane preparation. The former factor leads to the higher polymer concentration before solidification and thus denser top surfaces. The later factor leaves the enough time for the mass transport between polymer-rich phase and polymer-lean phase until the final solidification and leads to the more porous bottom surfaces. Furthermore, it seems that the bottom surfaces present the similar structures, which are resulted from the heat preservation of the stainless steel plate in membrane preparation process.
M-3
M-2
M-1
M-4
M-5
8
Fig.2 The top surfaces of PVDF/SiO2@GO hybrid membranes
M-1
M-2
M-3
M-4
M-5
Fig.3 The bottom surfaces of PVDF/SiO2@GO hybrid membranes
M-1
M-2
M-3
9
M-4
M-5
Overall
Near top surface
Near bottom surface
Fig.4 The cross-sectional morphologies of PVDF/SiO2@GO hybrid membranes M-4
M-5 M-1
Fig.5 The EDS mapping of the M-4 and M-5(Si mapping) As shown in Fig.4, the cross-sectional morphologies of PVDF/SiO2@GO hybrid membranes present the asymmetric bi-continuous sponge-like structures without spherulites, which can further demonstrate that the membranes are dominated by liquid-liquid phase separation mechanism. This structure provides the base for high permeability and high mechanical strength of the membranes[16]. Furthermore it is interesting that the part near the top surface of membrane becomes denser and the part near the bottom surface turns to be looser with the loading of SiO2@GO, which have been discussed above. Moreover, the formation mechanism asymmetric bi-continuous sponge-like structures without spherulites could be analyzed by nucleation and growth of PVDF crystal induced by a TIPS process. In details, three principal accounts are involved: (i) liquid-liquid phase separation is initiated by temperature gap; meanwhile the loaded SiO2@GO nanohybrid acts as nucleuses and induce the nucleation and growth of PVDF crystals; (ii) upon substantial increasing the addition of SiO2@GO nanohybrid, more nucleus favors the formation of more crystalline grains, and consequently produces the smaller pores onto the top surface; (iii) overhigh SiO2@GO content brings about time shortage to crystal growing owing to viscosity increase. Hence the crystalline decreases and causes the 10
bigger pores and higher porosity. The FESEM images of top surfaces in Fig. 1 also prove the hypothesis above. Xu et al.[10] also observed the similar phenomena in PVDF membranes blended with O-MWNTs via TIPS method. Cui et al.[11] verified the effects of nucleation and growth on the structure and performance PVDF membranes, which were prepared by the ternary mixture of PVDF/SiO2/DBP via TIPS process. The results also demonstrated that the membrane surface became denser with the addition of micro-particals. The dispersion of SiO2@GO in membrane matrix also influences the morphology and membrane performances. Fig.5 illustrates that Si species is homogeneously dispersed in the cross-sectional part of membranes due to good compatibility.
Fig.6 AFM images of the top surfaces of PVDF/SiO2@GO nanohybrid membranes 11
Table 2 The surface roughness parameters of PVDF/SiO2@GO nanohybrid membranes Membrane no.
Ra(nm)
Rq(nm)
Rmax(nm)
M-1 M-2 M-3 M-4 M-5
41.223 23.247 17.588 17.975 20.082
52.951 29.65 22.101 22.822 25.646
368.24 238.93 165.18 179.72 209.17
The top surfaces are also detected by AFM technique and shown in Fig.6. According to the mean roughness (Ra and Rq) and maximum roughness (Rmax) in Table 2, the surfaces of nanohybrid membranes gradually change to be smoother with the addition of SiO2@GO compared to the neat membrane M-1. However, membrane M-5 deviates from this trend. It becomes much rougher with 1.2wt.% SiO2@GO in the dope, but still show smoother surface than that of neat membrane M-1. The roughness difference should be contributed to the nucleation and growth of polymer induced by the addition of SiO2@GO. Up to the 0.9wt.%(membrane M-4), the added SiO2@GO nano fillers induce much more nucleuses and smaller crystal grains, which leads to the smoother top surface. However, when SiO2@GO content attains 1.2wt.% (membrane M-5), the viscosity of cast solution dominates the phase separation process[35]. From Table 2, the results of AFM images of membranes are consistent with the FESEM images in Fig.2. 3.3 The hydrophilicity and static protein adsorption of PVDF/SiO2@GO hybrid membranes
110 105 100
Contact angle()
95 90 85 80 75 70
M-1 M-2 M-3 M-4 M-5
65 60 55 50 45 0
100
200
300
400
500
600
Time(s) Fig.7 The dynamic contact angles of PVDF/SiO2@GO nanohybrid membranes
12
The surface hydrophilicity of PVDF/SiO2@GO hybrid membranes is detected by the dynamic contact angle of water drop. As shown in Fig.7, the surface contact angle evidently decreases with the loading of SiO2@GO in the membranes, indicating the increase of surface hydrophilicity[36]. Therefore, the nanohybrid membranes with SiO2@GO loading should get higher antifouling ability [36-38]. From Fig.7, it also can be seen that the surface contact angle of M-5 decrease much more quickly than other samples. It may be result of the much larger pores on the surface, which promotes the penetration of water drop into membrane matrix.
138.7
137.1
140
M-1
M-2
108.6
2
BSA absorption capacity(g/cm )
160
120
74.7
100
62.4
80
60
M-3
M-4
M-5
Fig.8 The static absorption capacities of PVDF/SiO2@GO nanohybrid membranes for BSA aqueous solution. To investigate the protein resistance, the static absorption properties of PVDF/SiO2@GO nanohybrid membranes for BSA aqueous solution are carried out. The results are illustrated in Fig.8. It can be seen that the static absorption capacity of nanohybrid membranes for BSA protein clearly decreases with the loading of SiO2@GO. A BSA adsorption capacity of 138.7±1.2 μg/cm2 is obtained for the bare PVDF membrane (M-1), which gradually decreases to 74.7±0.5 μg/cm2 and 62.4±1.2 μg/cm2 for membrane M-4 and membrane M-5 respectively. Obviously the nanohybrid membranes containing more SiO2@GO exhibit lower absorption capacity, which will improve the protein fouling ability of membranes in filtration process. The variation of static absorption for BSA is well accordance with the hydrophilicity of PVDF/SiO2@GO hybrid membranes. The previous work[10] [39, 40]also demonstrated that hydrophilicity enhancement can effectively mitigate the
13
protein adsorption of membranes. 3.4 The permeation performances of PVDF/SiO2@GO nanohybrid membranes Table 3 The flux(JW1), BSA rejection(R)，melting temperature(Tm) and crystallinity(xc) of PVDF/SiO2@GO nanohybrid membranes Membrane no.
JW1(L·m-2·h-1·bar-1)
R (%)
Tm(°C)
xc(%)
M-1 M-2 M-3 M-4 M-5
268.5±10.5 220.1±9.4 192.0±8.4 182.6±5.6 679.1±20.5
63.5±0.3 72.6±0.2 83.8±0.2 91.7±0.4 34.6±1.2
162.9 163.4 164.0 164.6 163.3
44.8 45.4 47.2 47.6 45.2
Solute separation and solvent permeation, as two essential properties of membranes, are estimated by BSA rejection and pure water permeation flux. It is well known that the BSA rejection and water flux are correlated with the pore size, porosity, surface properties and membrane structure. As shown in Table 3, compared to the bare PVDF membrane M-1, the pure water flux of nanohybrid membranes slowly falls off, and the BSA rejection evidently increases to 91.7% with the addition of SiO2@GO up to 0.9wt.% in the dopes. Nevertheless, compared with other membranes, the flux of membrane M-5 markedly increases about three times and attains 679.1 L·m-2·h-1·bar-1 accompanied a much lower BSA rejection. Although many factors including the mean pore size, porosity and surface hydrophilicity influence the permeability of membranes, permeation theory predicts that the filtration rate is directly proportional to the square of the effective pore size of the membrane. The nanohybrid membranes are demonstrated with the smaller pore size, denser top surface and thus the lower permeation flux despite they shows better hydrophilicity with the loading of SiO2@GO. Obviously, the flux improvement of membrane M-5 is attributed to its larger pore size and better hydrophilicity. What’s more, according to the water and the BSA rejection, the M-4 and M-5 show the typical ultrafiltration and microfiltration performances respectively. For the pressure-driven filtration membranes, such as ultrafiltration which only allows solute with certain size to pass through effective pore of membrane surface, size-sieving is primary factor influencing on the solute separation[10, 41]. It is really amazed that the large increase in permeability and decrease in BSA rejection from membrane M-4 to membrane M-5, which will be further investigated. Nevertheless, the overhigh viscosity of cast dope may be one of the reasons, which mainly due to the GO@SiO2 additive increases and blocks the movement of polymer chains, bring the higher porosity and bigger surface pore size. Above disscusion verifies that SiO2@GO addition evidently influences the permeation performances of PVDF/SiO2@GO nanohybrid membranes. However, from Fig.S2,
14
prinstine GO(without decoration of silica) addition has little effects on the permeation flux and BSA rejection of PVDF/GO hybrid membranes, which is accordance with the top morphology. 3.5 Long- term stability test of PVDF/SiO2@GO nanohybrid membranes Although polymeric membranes gain wide acceptance due to the simple operation, low cost and acceptable discharge quality, they are usually faced with the serious fouling and flux decline due to absorption and/or pore plugging, which leads to the productivity decline and increase of operation cost[42, 43]. In order to clearly investigate long-term stability test of PVDF/SiO2@GO hybrid membranes, two cycles of dynamic filtration test are addressed below. The membrane fluxes as a function of time are illustrated in Fig.9. During the BSA filtration process, the permeation flux of protein solution declines and then gets stable after 30 min filtration, which is resulted from the protein adsorption on membrane surface, pore plugging and concentration polarization etc. After washing with NaClO aqueous solution for 30min, the pure water flux rebounds in different degrees. To quantitatively evaluate the anti-fouling property of membranes, the reversible fouling ratio (Rr), irreversible fouling ratio (Rir) and total fouling ratio (Rt) and the flux recovery ratio (FRR) parameters of each membrane are calculated and presented in Fig. 10. It can be seen that the Rt and Rir of bare PVDF membrane M-1 are 72.9% and 52.5%, respectively. However, the nanohybrid membranes present a falling tendency in both Rt and Rir. Especially for membrane M-5, the Rir value even declines to 3.4%, which means the water flux can almost be recovered completely. Moreover, the FRR values of the nanohybrid membranes increase with the addition of SiO2@GO nanosheets despite of the membrane surface structure, which is positively correlation with the surface hydrophilicity of membranes. Depending on the hydrophilicity of membrane surface and the results of dynamic filtration, it can be concluded that the addition of SiO2@GO nanohybrid in the membranes enhances the antifouling performances.
15
700
M-1 M-2 M-3 M-4 M-5
Jw1 600
-1
Flux(Lm h bar )
500
-2
-1
400
300
200
100
BSA
0 0
NaClO
20
NaClO
BSA
40
60
80
100
120
Time(min)
Fig. 9 Time-dependent fluxes of PVDF/SiO2@GO nanohybrid membranes (M-1~M-5)
100
FRR(%)
Rt(%)
Rr(%)
Rir(%)
Percentage(%)
80
60
40
20
0 M-1
M-2
M-3
M-4
M-5
Fig.10 The fouling parameters of PVDF/SiO2@GO nanohybrid membranes
16
3.6 The crystallization behaviors of PVDF/SiO2@GO nanohybrid membranes In general, PVDF chains can crystallize into at least three distinct forms: α, β and γ crystalline phases. In order to measure the crystallation properties of as-prepared membranes, WXRD pattern is applied to inspect the PVDF/SiO2@GO nanohybrid membranes. Four striking diffraction peaks appear at 17.5°,18.4°, 19.9° and 26.5°, which correspond to the reflections of (100), (020), (110), (021) planes of α-phase with a trans-gauche (TGTG) PVDF crystals[44]. Generally, high temperature favors the formation of α-phase and low temperature induces β-phase [9, 45]. The WXRD patterns of PVDF/SiO2@GO nanohybrid membranes shown in Fig. S3 well accord with this regularity. Furthermore, despite its very low content of SiO2@GO in the membranes, the newly obtuse peak at about 23~24° demonstrates the existence of amorphous silica coating [31, 46], which is
764
795
872 855 840
974
the apparent evidence of SiO2@GO.
M-1
Transmittance(a.u.)
M-2 M-3 M-4 M-5 Si-OH
1000 -1
Wavenumber(cm )
Fig. 11 The ATR-FTIR spectra of PVDF/SiO2@GO nanohybrid membranes To further examine the polymorphism of PVDF, the ATR-FTIR spectra of the hybrid membranes were performed. As shown in Fig.12, the bands at 974, 872, 855, 795 and 764 cm-1 are the characteristic bands of α-phase PVDF [1, 44, 47]. Once SiO2@GO is introduced into the matrix, a weak band appears at 840 cm-1 for the nanohybrid membranes (M-2~M-5), which is ascribed to β-phase of PVDF [4, 44, 48]. From Fig. 12, compared to the neat membrane of M-1, the spectra of the nanohybrid membranes show a weak band at
17
950cm-1 (νSi-OH), which ascertains that SiO2@GO is successfully integrated into the hybrid membranes. The DSC curves of PVDF/SiO2@GO nannohybrid membranes with different additive contents are presented in Fig. S4, and the melting temperature(Tm) and crystallinity(xc) of membranes are listed in Table 3. Compared to the neat membrane M-1, the melting temperature and crystallinity of hybrid membranes initially increase with the content of SiO2@GO up to 0.9wt.% for membrane M-4, and then decline as the SiO2@GO concentration is further increased. These behaviors can be interpreted that SiO2@GO nano fillers in the matrix results in the combined effect of nucleation and growth of PVDF crystal and viscosity increase of cast solution. The appropriate amount of additive can prompt the PVDF crystallization; at the same time the viscosity increase of cast solution inhibits crystal growth on the contrary. Consequently, the membrane crystallization trend agrees with hypothesis of membrane mechanism above. 4. Conclusions The derivative of GO, SiO2@GO nanohybrid, is fabricated and employed to synthesize PVDF/SiO2@GO nanohybrid membranes via TIPS method for the first time. The following conclusions can be drawn: (1) The synthesized PVDF/SiO2@GO nanohybrid membranes experience liquid-liquid phase separation mechanism, and exhibit bi-continual and asymmetric structure as well as outstanding fibrous top surface and sheaf-like bottom surface. The included SiO2@GO nanohybrid is uniformly dispersed in the membrane matrix. With the addition of SiO2@GO, the top surface becomes denser and the pore size decreases; but overhigh SiO2@GO addition for M-5 triggers in the adverse trend. This is caused by the combined actions of nucleation and growth of PVDF and viscosity increase of cast solution due to the addition SiO2@GO nanohybrid. (2) The BSA rejection of membranes gradually increases with the addition of PVDF/SiO2@GO nanohybrid accompanied with the decline of pure water permeation flux. As the SiO2@GO content increases to 0.9wt.%, the PVDF/SiO2@GO nanohybrid membrane M-4 presents the highest BSA rejection of 91.7% and the lowest permeation flux of 182.6 L·m-2·h-1·bar-1. However, the overhigh SiO2@GO addition (1.2wt.%) leads to the outstanding pure water permeation flux of 679.1 L·m-2·h-1·bar-1 and a much lower BSA rejection. The permeation flux and BSA rejection are dominatedly related with the pore size despite the variations of other parameters. (3) With the addition of PVDF/SiO2@GO nanohybrid, the resultant membranes exhibit a lower BSA adsorption tend, an improved surface hydrophilicity and antifouling ability. (4) The XRD patterns and FTIR spectra of membranes exhibit the exclusive α-phase of PVDF, and the 18
melting temperature(Tm) and crystallinity(xc) evidently increase with the addition of PVDF/SiO2@GO nanohybrid up to 0.9wt.% in the dopes. Overhigh SiO2@GO addition (1.2wt.%) leads to small decline for these parameters.
Acknowledgements The research is supported by Science and Technology Commission of Shanghai Municipality (13ZR1429900, 14520502900) and International Joint Laboratory on Resource Chemistry (IJLRC).
Nomenclature JW1 pure water permeability (L·m-2·h-1·bar-1). Jw2 pure water permeability after washing with NaClO solution (L·m-2·h-1·bar-1). Jp1 The BSA aqeous permeability(L·m-2·h-1·bar-1). R
the rejection for solute (%).
V
the volume of pure water (L).
∆P the transmembrane pressure (bar/MPa)). A
the effective membrane area (m-2).
CP the concentrations of permeate (g/L). CF the concentrations of feed solutions (g/L). FRR
the flux recovery ratio (%).
Rt
total fouling ratio (%)
Rr
the reversible fouling ratio(%).
Rir the irreversible fouling ratio (%). xc ∆Hf
the crystallinity of membranes(%) The melting enthalpy of membranes (J/g)
∆Hf* The melting enthalpy of the neat PVDF(104.7J/g) Tm The melting temperature of samples(°C)
References [1] F. Liu, N.A. Hashim, Y. Liu, M.R.M. Abed, K. Li, Progress in the production and modification of PVDF membranes, Journal of Membrane Science, 375 (2011) 1-27. [2] X. Huang, W. Wang, Y. Liu, H. Wang, Z. Zhang, W. Fan, L. Li, Treatment of oily waste water by PVP 19
grafted PVDF ultrafiltration membranes, Chemical Engineering Journal, 273 (2015) 421-429. [3] G.-d. Kang, Y.-m. Cao, Application and modification of poly(vinylidene fluoride) (PVDF) membranes – A review, Journal of Membrane Science, 463 (2014) 145-165. [4] C.Y. Lai, A. Groth, S. Gray, M. Duke, Impact of casting conditions on PVDF/nanoclay nanocomposite membrane properties, Chemical Engineering Journal, 267 (2015) 73-85. [5] S. Berghmans, H. Berghmans, H.E.H. Meijer, Spinning of hollow porous fibres via the TIPS mechanism, Journal of Membrane Science, 116 (1996) 171-189. [6] J.A. de Lima, M.I. Felisberti, Porous polymer structures obtained via the TIPS process from EVOH/PMMA/DMF solutions, Journal of Membrane Science, 344 (2009) 237-243. [7] Z. Yang, P. Li, L. Xie, Z. Wang, S.-C. Wang, Preparation of iPP hollow-fiber microporous membranes via thermally induced phase separation with co-solvents of DBP and DOP, Desalination, 192 (2006) 168-181. [8] Z. Cui, N.T. Hassankiadeh, S.Y. Lee, J.M. Lee, K.T. Woo, A. Sanguineti, V. Arcella, Y.M. Lee, E. Drioli, Poly(vinylidene fluoride) membrane preparation with an environmental diluent via thermally induced phase separation, Journal of Membrane Science, 444 (2013) 223-236. [9] S.-i. Sawada, C. Ursino, F. Galiano, S. Simone, E. Drioli, A. Figoli, Effect of citrate-based non-toxic solvents on poly(vinylidene fluoride) membrane preparation via thermally induced phase separation, Journal of Membrane Science, 493 (2015) 232-242. [10] H.-P. Xu, W.-Z. Lang, X. Yan, X. Zhang, Y.-J. Guo, Preparation and characterizations of poly(vinylidene fluoride)/oxidized multi-wall carbon nanotube membranes with bi-continuous structure by thermally induced phase separation method, Journal of Membrane Science, 467 (2014) 142-152. [11] A. Cui, Z. Liu, C. Xiao, Y. Zhang, Effect of micro-sized SiO2-particle on the performance of PVDF blend membranes via TIPS, Journal of Membrane Science, 360 (2010) 259-264. [12] P.M. Atkinson, D.R. Lloyd, Anisotropic flat sheet membrane formation via TIPS: thermal effects, Journal of Membrane Science, 171 (2000) 1-18. [13] P.M. Atkinson, D.R. Lloyd, Anisotropic flat sheet membrane formation via TIPS: atmospheric convection and polymer molecular weight effects, Journal of Membrane Science, 175 (2000) 225-238. [14] G.-L. Ji, L.-P. Zhu, B.-K. Zhu, C.-F. Zhang, Y.-Y. Xu, Structure formation and characterization of PVDF hollow fiber membrane prepared via TIPS with diluent mixture, Journal of Membrane Science, 319 (2008) 264-270. [15] N.T. Hassankiadeh, Z. Cui, J.H. Kim, D.W. Shin, A. Sanguineti, V. Arcella, Y.M. Lee, E. Drioli, PVDF hollow fiber membranes prepared from green diluent via thermally induced phase separation: Effect of PVDF molecular weight, Journal of Membrane Science, 471 (2014) 237-246. [16] R. Carr, I.A. Weinstock, A. Sivaprasadarao, A. Muller, A. Aksimentiev, Poly(vinylidene fluoride) membrane preparation with an environmental diluent via thermally induced phase separation, Journal of Membrane Science, 8 (2008) 3916. [17] Y.-R. Qiu, H. Matsuyama, Preparation and characterization of poly(vinyl butyral) hollow fiber membrane via thermally induced phase separation with diluent polyethylene glycol 200, Desalination, 257 (2010) 117-123. [18] T. Xiao, P. Wang, X. Yang, X. Cai, J. Lu, Fabrication and characterization of novel asymmetric polyvinylidene fluoride (PVDF) membranes by the nonsolvent thermally induced phase separation (NTIPS) method for membrane distillation applications, Journal of Membrane Science, 489 (2015) 160-174. [19] X. Li, X. Lu, Morphology of polyvinylidene fluoride and its blend in thermally induced phase separation process, Journal of Applied Polymer Science, 101 (2006) 2944-2952. [20] H.B. Yuxi Xu, Gewu Lu, Chun Li, and Gaoquan Shi, Flexible Graphene Films via the Filtration of 20
Water-Soluble Noncovalent Functionalized Graphene Sheets, Journal of the American Chemical Society, 130 (2008) 5856-5857. [21] H. Li, S. Pang, S. Wu, X. Feng, K. Mullen, C. Bubeck, Layer-by-layer assembly and UV photoreduction of graphene-polyoxometalate composite films for electronics, Journal of the American Chemical Society, 133 (2011) 9423-9429. [22] S. Lee, B.G. Choi, D. Choi, H.S. Park, Nanoindentation of annealed Nafion/sulfonated graphene oxide nanocomposite membranes for the measurement of mechanical properties, Journal of Membrane Science, 451 (2014) 40-45. [23] J. Lee, H.-R. Chae, Y.J. Won, K. Lee, C.-H. Lee, H.H. Lee, I.-C. Kim, J.-m. Lee, Graphene oxide nanoplatelets composite membrane with hydrophilic and antifouling properties for wastewater treatment, Journal of Membrane Science, 448 (2013) 223-230. [24] H. Wu, B. Tang, P. Wu, Development of novel SiO2–GO nanohybrid/polysulfone membrane with enhanced performance, Journal of Membrane Science, 451 (2014) 94-102. [25] N. Wang, S. Ji, J. Li, R. Zhang, G. Zhang, Poly(vinyl alcohol)–graphene oxide nanohybrid “pore-filling” membrane for pervaporation of toluene/n-heptane mixtures, Journal of Membrane Science, 455 (2014) 113-120. [26] Z. Xu, J. Zhang, M. Shan, Y. Li, B. Li, J. Niu, B. Zhou, X. Qian, Organosilane-functionalized graphene oxide for enhanced antifouling and mechanical properties of polyvinylidene fluoride ultrafiltration membranes, Journal of Membrane Science, 458 (2014) 1-13. [27] Y.P. Tang, D.R. Paul, T.S. Chung, Free-standing graphene oxide thin films assembled by a pressurized ultrafiltration method for dehydration of ethanol, Journal of Membrane Science, 458 (2014) 199-208. [28] H.M. Hegab, L. Zou, Graphene oxide-assisted membranes: Fabrication and potential applications in desalination and water purification, Journal of Membrane Science, 484 (2015) 95-106. [29] W.S. Hummers, R.E. Offeman, Preparation of Graphitic Oxide, Journal of the American Chemical Society, 80 (1958) 1339-1339. [30] D.C. Marcano, D.V. Kosynkin, J.M. Berlin, A. Sinitskii, Z. Sun, A. Slesarev, L.B. Alemany, W. Lu, J.M. Tour, Improved synthesis of graphene oxide, ACS Nano, 4 (2010) 4806-4814. [31] L. Kou, C. Gao, Making silica nanoparticle-covered graphene oxide nanohybrids as general building blocks for large-area superhydrophilic coatings, Nanoscale, 3 (2011) 519-528. [32] X. Li, H. Liu, C. Xiao, S. Ma, X. Zhao, Effect of take-up speed on polyvinylidene fluoride hollow fiber membrane in a thermally induced phase separation process, Journal of Applied Polymer Science, 128 (2013) 1054-1060. [33] Y. Shen, A.C. Lua, Preparation and characterization of mixed matrix membranes based on PVDF and three inorganic fillers (fumed nonporous silica, zeolite 4A and mesoporous MCM-41) for gas separation, Chemical Engineering Journal, 192 (2012) 201-210. [34] K.G. Lee, R. Wi, M. Imran, T.J. Park, J. Lee, S.Y. Lee, D.H. Kim, Functionalization Effects of Single-Walled Carbon Nanotubes as Templates for the Synthesis of Silica Nanorods and Study of Growing Mechanism of Silica, ACS Nano, 4 (2010) 3933-3942. [35] J. Wang, W.-Z. Lang, H.-P. Xu, X. Zhang, Y.-J. Guo, Improved poly(vinyl butyral) hollow fiber membranes by embedding multi-walled carbon nanotube for the ultrafiltrations of bovine serum albumin and humic acid, Chemical Engineering Journal, 260 (2015) 90-98. [36] X. Zhang, W.-Z. Lang, X. Yan, Z.-Y. Lou, X.-F. Chen, Influences of the structure parameters of multi-walled carbon nanotubes(MWNTs) on PVDF/PFSA/O-MWNTs hollow fiber ultrafiltration membranes, Journal of Membrane Science, 499 (2016) 179-190. 21
[37] X. Li, M. Wang, C. Wang, C. Cheng, X. Wang, Facile Immobilization of Ag Nanocluster on Nanofibrous Membrane for Oil/Water Separation, ACS Applied Materials & Interfaces, 6 (2014) 15272-15282. [38] Y. Han, Y. Jiang, C. Gao, High-Flux Graphene Oxide Nanofiltration Membrane Intercalated by Carbon Nanotubes, ACS Applied Materials & Interfaces, 7 (2015) 8147-8155. [39] W.-Z. Lang, J.-P. Shen, Y.-X. Zhang, Y.-H. Yu, Y.-J. Guo, C.-X. Liu, Preparation and characterizations of charged poly(vinyl butyral) hollow fiber ultrafiltration membranes with perfluorosulfonic acid as additive, Journal of Membrane Science, 430 (2013) 1-10. [40] W.-Z. Lang, Q. Ji, J.-P. Shen, Y.-J. Guo, Z.-L. Xu, The roles of alkali metal counter-ions of PFSA play in the formation of PVDF/PFSA-M hollow fiber membranes, Desalination, 292 (2012) 45-52. [41] Z. Shi, W. Zhang, F. Zhang, X. Liu, D. Wang, J. Jin, L. Jiang, Ultrafast separation of emulsified oil/water mixtures by ultrathin free-standing single-walled carbon nanotube network films, Advanced materials, 25 (2013) 2422-2427. [42] S. Zhang, R. Wang, S. Zhang, G. Li, Y. Zhang, Development of phosphorylated silica nanotubes (PSNTs)/polyvinylidene fluoride (PVDF) composite membranes for wastewater treatment, Chemical Engineering Journal, 230 (2013) 260-271. [43] Y. Zhang, L. Wang, Y. Xu, ZrO2 solid superacid porous shell/void/TiO2 core particles (ZVT)/polyvinylidene fluoride (PVDF) composite membranes with anti-fouling performance for sewage treatment, Chemical Engineering Journal, 260 (2015) 258-268. [44] P. Martins, A.C. Lopes, S. Lanceros-Mendez, Electroactive phases of poly(vinylidene fluoride): Determination, processing and applications, Progress in Polymer Science, 39 (2014) 683-706. [45] I.J. Roh, S. Ramaswamy, W.B. Krantz, A.R. Greenberg, Poly(ethylene chlorotrifluoroethylene) membrane formation via thermally induced phase separation (TIPS), Journal of Membrane Science, 362 (2010) 211-220. [46] M. Obaid, G.M.K. Tolba, M. Motlak, O.A. Fadali, K.A. Khalil, A.A. Almajid, B. Kim, N.A.M. Barakat, Effective polysulfone-amorphous SiO2 NPs electrospun nanofiber membrane for high flux oil/water separation, Chemical Engineering Journal, 279 (2015) 631-638. [47] T. Boccaccio, A. Bottino, G. Capannelli, P. Piaggio, Characterization of PVDF membranes by vibrational spectroscopy, Journal of Membrane Science, 210 (2002) 315-329. [48] J. Wang, H. Li, J. Liu, Y. Duan, S. Jiang, S. Yan, On the α → β Transition of Carbon-Coated Highly Oriented PVDF Ultrathin Film Induced by Melt Recrystallization, Journal of the American Chemical Society, 125 (2003) 1496-1497.
Highlights:
SiO2@GO nanohybrids with high thermal stability are synthesized.
PVDF/SiO2@GO nanohybrid membranes are prepared via TIPS method.
SiO2@GO addition leads to permeation flux decline and BSA rejection increasing.
SiO2@GO addition improves the hydrophilicity and anti-fouling ability of membranes.
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